(1) Field of the Invention
The present invention relates generally to the determination of ship velocity, and more particularly to a method and system for estimating ship velocity through the water (i.e., relative to the water) and over the ground (i.e., relative to the ground).
(2) Description of the Prior Art
In shipboard active sonar systems, the change in received frequency (i.e., Doppler shift) based on relative speed of the ship is a critical issue when trying to differentiate moving targets from background (stationary) sources of reflection or reverberation. The Doppler shift in the echo from a stationary object is a function of the speed of the sonar's transmitter (i.e., ship velocity) through the water in the direction of the object. One device used to measure ship velocity is an electromagnetic velocimeter wherein a pitot tube is used to measure the speed of the ship in water. A change in pressure on the tip of the pitot tube is experienced due to the ship's speed in water which is measured by a corresponding change in its electrical resistance. However, this type of device is operationally unreliable, quickly drifts out of calibration and has a high error factor. Any errors in ship velocity lead to errors in Doppler compensation of the active sonar thereby degrading sonar and fire control performance.